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Subject: Re: Replication lag from transaction logs
From: Scott Ribe <scott_ribe@elevated-dev.com>
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Date: Mon, 18 Jun 2018 11:03:27 -0600
Cc: pgsql-admin@lists.postgresql.org
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To: Debraj Manna <subharaj.manna@gmail.com>
Precedence: bulk


> On Jun 18, 2018, at 9:56 AM, Debraj Manna <subharaj.manna@gmail.com> =
wrote:
>=20
> Thanks Keith this is useful.
>=20
> One more query if I need to know that if a have fallen too far behind =
and the WAL is not available. I guess I can do this. Let me know if my =
understanding is correct.
> 	=E2=80=A2 Run pg_controldata <DATA_DIR> on the slave node which =
has been down for long.
> 	=E2=80=A2 It will output the details about the WAL along with =
the WAL file name from where it will start the replication. (Field to =
look for in the output=E2=80=93 'Latest checkpoint's REDO WAL file')
> 	=E2=80=A2 Then check if the file mentioned in ` 'Latest =
checkpoint's REDO WAL file' is present in master `pg_wal` directory. If =
not then slave have fallen too far behind and will not be able to =
recover from WAL. =20

You could also try bringing the slave back up, and monitoring the log =
for the error about needed WAL file not being available--this avoids the =
race condition between checking that all WAL is available and restarting =
the slave.

--
Scott Ribe
scott_ribe@elevated-dev.com
https://www.linkedin.com/in/scottribe/